Age compositions of flatfish stocks as determined by a new otolithometric method, its application in the estimation of growth, spawning potential and fisheries management

Abstract

We developed an ageing methodology and examined age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. Ages were difficult to determine for three-lined tongue sole (Cynoglossus abbreviates) and ridged-eye flounder (Pleuronichthys cornutus) because the first year annulus ring was often indistinct; therefore, we used directional change in otolith growth to distinguish it. Sectioning and etching methods were powerful tools for identifying annual checks for red tongue sole (Cynoglossus joyneri). Using these ageing methods, we determined age–length relationships and growth curves. The age composition of the populations studied and of the landings showed that a large proportion of the latter consisted of individuals under the mean age of sexual maturity, thereby reducing the percent spawning potential ratio (%SPR) to ≈ 20% for all species. These findings suggest that fishing pressure on immature fish is leading to overfishing of these flatfish stocks.

Introduction

To maintain high levels of catch and stock abundance of flatfishes, effective fisheries management is essential. The persistence of a population requires sufficient spawning biomass, which means that year classes must produce sufficient spawning units per recruit (SPR) over their lifespan to correspond to the average number of recruits (R) produced by a unit of spawning (S). The percent spawning potential ratio (%SPR), which is expressed as a maximum biomass without fishing mortality, is useful for estimating the degree of recruitment overfishing. Although some objections to %SPR analysis exist (Rochet, 2000, Katsukawa, 2005), %SPR has begun to be calculated routinely and SPR criteria being given explicit consideration in fisheries management objectives (reviewed by Mace and Sissenwine, 1993). %SPR is usually estimated from cohort analysis (Virtual Population Analysis, VPA), but it is time-consuming and requires enormous effort. Thus, a simpler method of determining %SPR is greatly needed.

Year-class strength and age composition are the fundamental data used to calculate %SPR. In fisheries stock assessment, accurate and precise age information is necessary for the SPR analysis. Otolithometry is widely used for age determination, but identification of seasonal increments is complicated by false annuli and discontinuous structures that correspond to non-seasonal events. False annuli are non-periodic secondary zones that appear as translucent zones within opaque zones. Little is known about their cause, although temperature, oxidative stress, salinity, adverse weather, food intake, and developmental transitions have been implicated in their formation (Berghahn, 2000, Cappo et al., 2000, Wright et al., 2002). Williams and Bedford (1974) reported false annuli and checks, especially during spawning. Methodology for reading opaque and translucent zones is limited, and criteria for distinguishing between seasonal zones and false annuli do not exist.

In this study, we developed an otolithometric ageing method that can be used to examine age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. These species are commercially important to small trawl net and gill net fisheries. Flatfishes usually have distinguishable annuli on otoliths; however, tongue sole otoliths are invisible to surface reading and their first opaque annuli are difficult to observe with the otolith sectioning method. In this study, we first observed and detected an otolith inner structure, and we propose that this structure be used for age determination. We validated seasonal annulus formation, and we examined male and female age and growth and age compositions of the landings of three flatfish species. Second, we calculated %SPR of these flatfish populations using a simple method based on the age composition. Based on these procedures, we propose a simple and precise method of fish stock assessment for these flatfish resources and present the overfishing status of flatfish stocks in Seto Inland Sea.